Fix CI failures, upgrade to Go 1.14+

- Modify Travis manifest to use Go 1.14.x
- Hardcode version of golangci-lint in setup script
- Hardcode version of go-md2man in setup script

Signed-off-by: jdolitsky <393494+jdolitsky@users.noreply.github.com>

.travis.yml

@@ -7,7 +7,7 @@ services:
 language: go
 
 go:
-  - "1.13.x"
+  - "1.14.x"
 
 go_import_path: github.com/docker/distribution
 

script/setup/install-dev-tools

@@ -1,11 +1,17 @@
 #!/usr/bin/env bash
 
+GOLANGCI_LINT_VERSION="v1.27.0"
+GO_MD2MAN_VERSION="v1.0.10"
+
 #
 # Install developer tools to $GOBIN (or $GOPATH/bin if unset)
 #
 set -eu -o pipefail
 
+# Enable Go modules
+export GO111MODULE=on
+
 # prevent updating go.mod of the project
 cd /tmp
-go get -u github.com/golangci/golangci-lint/cmd/golangci-lint
-go get -u github.com/cpuguy83/go-md2man
+go get "github.com/golangci/golangci-lint/cmd/golangci-lint@${GOLANGCI_LINT_VERSION}"
+go get "github.com/cpuguy83/go-md2man@${GO_MD2MAN_VERSION}"